Cathode-ray tube arc-over preventive



March 13, 1951 L. E. SWEDLUND 2,545,120

CATHODE-RAY TUBE ARC-OVER PREVENTIVE Filed Feb. 27, 1948 2 Sheets-Sheet 1 INVENTOR LL DYD E. SW5 DLUND A TzN EY March 13, 1951 E. SWEDLUND CATHODE-RAY TUBE ARC-0VER PREVENTIVE 2 Sheets-Sheet 2 Filed Feb. 27, 1948 Int INVENTOR LLOYD ESWEDLUND,

ATTO

Patented Mar. 13, 1951 CATHODE-RAY TUBE ARC-OVER, PREVENTIVE Lloyd E. S'wedlund', Lancaster, Pa., assignor to Radio Corporation of America, a; corporation of Delaware Application February 2'7, 1948; Serial 'No. 11,623

13Claims. (Cl. 250-141) My invention relates to cathode ray tubes and in particular to a means to suppressarc-overs between electrodes maintained at high diiferences of potential during tube operation.

Cathode ray tubes, used in one form. of projection type television receivers, utilize an electron gun structure having a second anodeor accelerating electrode maintained at close t;27,000

volts positive relative to the reference or cathode potential during tube operation. The second anode electrode in these-tubes may comprise a conductive internal bulb coating, withwhich the possibility of breakdown or arc-over. tolower voltage electrodes exists. Breakdown or arc-over within the tube is a problem on higher voltage tubes particularly in the region between the second anode coating and th end; of a first anode electrode. A high gradient appears inthis region and internal arc-over may occur. Also, a high gradient appears near the edge of the second anode coating or at the edge of thegroundedexternal neck coating which serves-as a sh eld between the defiecting yoke and the tubeif it does not overlap the internal coating. This non-uniform gradient tends to produce failure-near t e second anode coating edge. Edge-break-down failure such as this is well known from electrical break-down tests on glass and other insulators.

It has been observed that in ernal arcsthrough the vacuum space do not usually'produce permanent damage providing the peak arc'current is limited. Under these conditions the arc-over is mom ntary in most cases. However, it is, not usually possible to limit the current enough with- .out resulting in poor vo tage regulation of the second anode supply sothat the arc-overs are sustained and often overheat internal parts, causing excessive gas and fracture of the glass envelope.

It is therefore an object of. my invention. to provide means for improving the: operation of cathode ray tubes.

It is a further object of my inventionto provide means for preventing electrical breakdown between elements of a cathode ray tube.

It is another object of my invention to-prov-ide means for suppressing arc-overs between elements of a cathode ray tube.v

It is a further object of my invention to provide means for suppressing arc-overs between electrodes of a cathode ray-tube maintained at a high difference of potential. g

It is a further object of my invention. to provide means for suppressing arc-overs betweena 2. conductive wall coating and adjacent tube elements.

It is also an object of my invention to provide means for suppressing arc-overs between a. conductive wall coating of a cathode ray tube and adjacent electrodes.

It is a further object of my invention to provide means for suppressing corona discharge between an internal conductive coating of acathode ray tube and an adjacent external tube element.

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims, but the invention itself will best be understood by reference to the following description taken in connection with the accompanyingdrawing, in which:

Figure l is across sectional view of a cathode ray tube incorporating myinvention;

Figure 2 is an enlarged partial view of a conventional cathode ray tube similar, to. that. of Figure 1;

Figure 3. is a sectional, view of. a. cathode ray tube incorporating a modification, of my invention: and c Figure 4 is a cathode ray tube incorporating 7 another modification of my invention.

Referring to Figure 1, there is disclosed, a cathode ray tube of the projection type for reproducing television images. The illustrated tube consists of aconventional. glass or similar vitreouselongated envelope it! having a cone or bulb shaped end H and an elongated. neck portion 18. Closing the bulb end. of, the envelope is a glass face plate I 2, on the inner surface of which is deposited a phosphor screen. I4, covered by a thin aluminum metal screen I6. Within the neck portion [8' of the receiving tube is provided an. electrongun structure for generating and. focusing a beam of electron-5 upon the phosphor screen, I4. This electron gun structure comprisesa. tubular support 20 mounted coaxially within the envelope portion I 8. The support 20. encloses anindirectly heated, oxide coated cathode (not shown) which. serves as'the source for the, beam electrons. The control grid (also not shown)- is in the formof an apertured plate spaced a few mils from the activated cathode-surfaceand closing the end. of. the tubular support 20- facing. the fluorescent screen. Coaxially spaced from. the control grid. is a shield electrode 22 comprising a short cylindrical tube supporting a: transverse apertured disc (not shown). The shield electrode 22 is normally operated in onetype oftube at a positivepotential relative-to cathode potential of approximately 200 volts,v Electrode 22 serves to prevent interaction between the fields of the control grid and the focusing fields of the anode electrodes described below.

A tubular first anode electrode 24 is axially spaced as is shown from the second grid 22. The first anode electrode is normally operated in the mentioned type of tube at around 4800 volts. The lens formed between the shield electrode 22 and the first anode 24 aids in the focusing of the electrons passing through the aperture of the screen electrode 22. A second lens is formed between the first anode electrode 24 and a second anode electrode, which comprises conductive graphite coating 26 on the inner surface of the envelope neck Hi. The second anode coating 26 extends as is shown into the bulb portion l l and makes contact with the aluminum screen I5. The lens between the first anode electrode 24 and the second anode electrode 26 provides a final focus to the electrons passing through the tubular electrode 24 and forms them. into a beam 21 focused upon the fluorescent screen l4. lhe second anode electrode 26 may be operated in the one typ of the tube, mentioned above, at around 27,000 volts. The appropriate potentials are maintained on the several electrodes described above by leads sealed through the closed end of the envelope neck portion l3. These leads are in turn connected to base pins 30 fixed into the tube base 28. external circuit (not shown) through a lead conductor 32. The lead 32 may comprise a metal cavity cup sealed through the wall of the envelope bulb H and in contact with the graphite conductive coating 26 of the second anode.

The electron beam 21 is caused to scan over the surface of the fluorescent screen Hi by either well known electrostatic or electromagnetic deflection movements. Figure 1 shows electromagnetic scanning means at 34 which is a neck yoke including coils that produce fields at right angles to each other and to the axis of the tube. These coils would be connected to the usual saw-tooth or. other variable current sources of line and frame frequency (not shown).

In tubes, of the type described above, the second anode electrode 26 is operated at a positive potential up to 30,000 volts. Under such conditions, field emission between the first and second anode electrodes, becomes a serious problem. Besides reducing contrast by random illumination of the screen, fairly well focused beams of field emitted electrons impinging on the neck [8 adjacent the first anode thimble 25 cause localized heating which result in punctured and cracked necks. As illustrated in Figure 2, in a conventional tube there is a high field gradient between the first anode electrode 24 and the second anode coating 26 on the adjacent neck portion. Internal arc-overs usually occur in this region due to several things. A concentration of field emission will produce a heating of the glass tube wall to release occluded gas therefrom which will support an arc-over. Also, any roughness of, or projections from the conductive graphite coating 26 will form paths of less resistance for the field emission. The field emission will seek out such points and will concentrate at these points to form a hot spot.

There is also another field concentration, disclosed in Figure 2, between the end of the conductive coating 26 and both the thimble 25 and the grounded conductive coating 35 on the outside surface of the tube bulb. Since both the grounded coating 36 and the first anode electrode The second anode 26 is connected to the 24 overlap the end of the conductive coating 26, this field concentration or end-effect not only encourages arc-over between the electrode 24 and the high potential coating 26 but also tends to cause puncture of the tube neck. This results from the fact that the field concentration produces a non-uniform potential gradient in the glass wall, thus increasing unit stress. This increased gradient also ac.s to produce field emission at the end of the coating and field emission tends to heat the glass walls of the tube, and in accordance with thermal breakdown theory which is well supported by experiment with glass, this leads to electrical puncture or breakage of the glass. I have observed that arc-overs between the first and second anode electrodes do not usually produce permanent damage if the peak current is limited. In fact when the current is limited, the arc-over is only momentary in most cases. However, it is not usually possible to limit the current of the arc-over enough without causing poor voltage regulation in the supply circuits of the anode electrodes.

It is often possible to burn out or remove field emission by a process known as spot knocking. The electrodes are subjected to about twice the normal operating voltage in series with a limiting resistor. Under favorable conditions enough current can be allowed to flow, during arc-over, to fuse or melt off the sharp points or irregularities causing field emission. In practice this is not always successful, and the current is likely to be raised high enough to produce a destructive arc. This process not only requires time and special equipment for tube manufacture but also requires skill and good judgement to avoid an excessive number of ruined tubes. In addition it has been observed that the tubes sometimes contain a small amount of' fine loose particles which may move and settle at a point of high gradient after the tube is handled. This may then result in the recurrence of field emission at operating voltages. Whereas spot knocking was required on nearly all of the tubes of old construction and a number of them were damaged, my invention practically avoids the need for it and, in the few cases where it is needed, does not result in tube failure.

As a solution of the problems outlined above, I have provided, as shown in Figures 1 and 3, a means for suppressing internal arc-overs within the tube to avoid damage incident to such electrical breakdown. My novel solution is to pr0 vide a semi-conducting coating 38 in the region of the first anode electrode 24. The conductive coating 26 is stopped short of the first anode electrode thimble 25 and the second anode is continued as a semi-conducting coating 38 to a. point on the neck wall overlapping the first anode thimble 25. This strip of coating material 38 is still part of the second anode electrode and, as will be pointed out, provides a focusing and accelerating field for the electron beam 27. The semi-conductive coating material 38 i preferably a mixture of aluminum oxide suspended in a sodium silicate solution. This material is applied wet to the inside of the neck wall and then baked until the water is driven off and the coating becomes a hard adherent substantially gas free layer. The aluminum oxide together with the sodium silicate provides a, high resistanoe or a very low conductivit layer. The coating material should have a desirable range of resistivity in the order of between 10 and 10 ohms. The resistance should be such that the 5. current tolerated not be morethanapprox imately 50 microchips-flowing through the-material; That; is, the conductivity of the coating 'm'ateriali 38 must be high enough to take care of stray field emission and leakage currents between the two electrode 24- and 2d so that these currents may be carried off and allow the area to return to an equilibrium potential within aboutone second.

When high voltage potential is applied to the anode coating 26, during tube operation, the conductivity of the coating 38" is sufficient to permit at. least the upper part of the coating 38' to assume the high potential. In case field emission is present, or due to an increase of voltage an arc is established between the first anode electrode and the second anode coating 38, the potential around the hot spot, which tends to develop on coating 38, will drop immediately before the current in the arc can build up and produce damaging effects. With the drop in voltage, ionization ceases and the arc stops. The drop in potential through the coating 38, around any point of concentration of field emission or are from the first anode, is due to the low conductivity of coating 38, which thus produces a large voltage drop even with a very small current. The use of the coating 38 prevents any concentration of heating, since the drop in potential at the point of concentration will cause the, discharge to spread out and over-heating of the glass parts of the tube will be prevented. The electrical. path that an arc discharge would normally take in this region of tube [0, is along alow resistance are between the first anode electrode 2'4 and the second anode coating 25, then along the high conductivity path of the second anode coating 26 to the second anode lead 32. The presence, however, of coating 38 provides a high resistance in series with the arc path and the high conductivity path of the coating 26. Thus, one function of such a high resistance portion 38 of the second anode electrode is to quench and remove any are discharge established between the first and second anode electrodes 24 and 26 respectively. It is obvious that the high conductivity portion of coatin 26 should be sufficiently spaced from the first anode electrode 24 to prevent direct arcing between the firs anode and the conductive coating 26.

The use of the low conductivity coating 38 also reduces the danger of an arc-over between 6a concentration of nel d beween the-end or the sec oncl anode coating 26 and the grounded-coating 3-6 (-F-igures 1' and- 21.

It is also characteristic of the electrical'break down of glass according to. the thermal breakdown theory, that leakage current. flows and tends-to concentrate in av narrow path, :due to heating of the glass which has a negative temperature coefficient of: resistance. The low con-- ductivity: coating 38 acts. to. suppress this current concentration the region most. susceptiblewto puncture.

An alternative application of my invention'is disclosed in Figure in. which a lowconductivity coating 42: is placed between two portions. and '26 of the second: anode electrode. The low conductivity coating 42 is applied preterably in the portion of the. tube where the neck begins toflare and form the bulb portion H. This arrangement also provides a highiresistance path 42 in the arcdischarg'e'path between the first anode electrode 24v and the second anode contact lead 32 similar to that as described above for Figure 1. The low conductivity coating 42 will prevent destructive arc-overs; since any arc-over which will develop between electrode: 24 and, the second anode coating-portion '41) will reduce the potential of portion 40: so that the voltage and current are decreased and the are suppressed. The conductivity of the coating strip 42 is =si=milar to that of coating 38 in the modification of Figure 1 and is such as. to permit the second anode portion 40 to regain its high potential within a'very short time after any arc-over.

Another advantage of positioningthe semiconductive coatin 42 at the beginning of the neck is that in this position, coating 42acts to reduceanyedge effect between the forward edge of the grounded outside neck coating 36 and the high potential coating 260m the inner surface of the tube. Thus, glasspuncture due to corona bethe grounded coating 36 and the second anode electrode 26, which would result in destructive puncturing of the glass neck wall as described above. In tubes of the type described relative to my invention, there is often provided a bulb spacer 23 (Figures 1 and 2) mounted on the first anode tube 24 and provided with fingers 29 for resiliently positioning the first anode tube 2-4 along the axis of the neck portion [8; The spacer 2'3, usually of a metallic material, will be at first anode potential during tube operation. Due to the use of high second anode potentials relative to that of the first anode 2'4, there will be current leakage across the glass surface of the neck [8 between the spring fingers 2s and the second anode coating 25 (Figure 2). The use of the low conductivity coating 38 (Figure 1) results in a field gradient being established inthe low conductivity coating 38 by this surface leakage from the spring fingers 29. Thispotential gradient in the coating 38 between the second anode coating 26 and the edge of coating 38 facing the fingers 29 will eliminate any dangerous tween the: yoke 34 or coating 36- and: the second anode coating2 6 isprevented.

Itis not necessary-to. use an internal neck coat-- ing on the bulb surfacegto provide; a resistance in the arc-over path, between the first and second anode electrodes. In Figure '4, the second anodeelectrode is deposited on the inside wall of the. tube in two portions; one portion being a conductive layer, 26, on the inner surface-of the bulb lit and asecond portion, All, deposited on theneck t8 or: the tube and. overlapping; the. first. anode electrode 24. The anode portions 26 and. do; are spaced along the inside sui -race of the tube so that there is a. space 44 of insulating glass between them. The second anode coating portion 2:61 is connected to its source of high potential through the second anode contact lead 32 and a conductive lead 45. Also th second anode portion llis. connected to the same source of high potential by a conductor 5e passing. through the glasswall of the neck. However; between the two anode portions 2% and ie is placed a high resistance element 43. The resistor 48 may have a value of between lllafiOd-to l0;000,900 chm-s; The resistance:- Afl functions. in a manner similar to the conductive coatings 3.2 and 48, described in the modificationsoi Figures 1 and 3, respectively. If the conditions are such, duringthe operation of the tube .ofFigure 4a, to establish a. concentration of the field emission and support: to. an. arcover between the firstanode electrode 24: and the second-anodeportion 4B, the presence of the resistor 443 in the circuit of the second anodeportion Mi will. result in: a drop in th potential of portion-40 belowfthat at which the arc 'can be sustained." j y It has been mentioned above that the coating material 38 and 40 may consist of a mixture of aluminum oxide powder in a suspension of sodium silicate. Howevenit is possible to obtain equally good results with other powders of insulating or semi-insulating material. For example, finely divided ferric oxide or chromic oxide may be substituted for the aluminum oxide. Any finely divided metal having surface layers of insulating or semi-insulating oxides, for example, iron, nickel and copper may be substituted for the metal oxide of the semi-conductive coatings 38 and 42. Also any other high resistance coating suitable for use in a vacuum tube may be used.

In'using a ferric oxide mixture, good results have been obtained by dissolving 100 to 125 grams of sodium silicate in 260 to 500 cc. of distilled water; grams of finely powdered ferric oxide (F8203) may be added slowly and with constant stirring. The proportions of the constituents can be varied considerably without impairing the'cffectiveness of the coating as long as the resistivity of the coating is approximately within the limits described above. This coating mixture may be applied to the inner surface of the glass envelope l 9 in any convenient way as by the use of a brush,

spray,"etc., and in as thin a layer as will produce the desired conductivity.

'While certain specific embodiments have been illustrated and described, it will be understood thatvarious changes and modifications may be made therein without departing from the spirit and scope of the invention.

- What I claim as new is:

1. A cathode ray tube comprising an envelope,

a first anode electrode mounted within said envelope and adapted to be maintained at an operational potential, said first anode electrode spaced from the wall of said envelope, a second anode electrode including a conductive coating on the inner surface of said wall, lead means connected to one portion of said second anode coating for connecting said second anode to a source of higher potential relative to that maintained on said first anode electrode during tube operation, and means of high electrical resistivity electrically connected to another portion of said second anode coating and between said first anode elec- The solution maybe heated and 500' trode and said one portion of said second anode coating.

2. An electron discharge device comprising an evacuated envelope, a first electrode mounted within said envelope and adapted. to be maintained at an operational potential, said electrode spaced from a portion of the wall of said envelope,

a second electrode including a conductive coating on-th inner surface of said wall portion, lead means sealed through said wall portion for connecting said conductive coating to a source of higher potential relative to that maintained on said first electrode during tube operation, said conductive coating including a semiconductive coating on the wall of said envelope and more adjacent said first electrode than said lead means, said semiconductive coating having high electrical resistance and in electrical contact with said second electrode to suppress electrical discharges between said first and second electrodes.

3. A cathode ray tube comprising an envelope, a first anode electrode mountzd within said envelope and adapted to be maintained at an operational potential, said anode electrode spaced from the wall of said envelope a second anode electrode including a conductive coating .on the inner surface of said wall, lead means sealed through said wall and in contact with said second anode coating for connecting said coating to a source of higher potential relative to that maintained on said first anode electrode during tube operation, said second anode coating including an area of high electrical resistance more adjacent said first anode electrode than said lead means for suppressing electrical discharges between said first and second anode electrode.

4. An electron discharge tube comprising an evacuated envelope, an electrode mounted within said envelope and adapted to be maintained at an operational potential, said electrode spaced from a portion of the wall of said envelope, a conductive coating on the inner surface of said wall portion, and conductive means for connecting' said conductive coating to a source of higher potential relative to that maintained on said electrode during tube operation, said conductive means including a resistor of low electrical conductivity for suppressing'an electrical discharge between said electrode and said conductive coat mg. l

5. A cathode ray tube comprising an evacuated envelope, a first anode electrode mounted within said envelope and adapted .to be maintained at an oprrational potential, said anode electrode spaced from the wall of said envelope, a second anode electrode including a conductive coating on the inner surface of said wall and conductive means for connecting said conductive coating to a source of h gh potential relative to that maintained on said first-anode electrode during tube operation, said means including a resistor of low el:ctrical conductivity for suppressing an'electrical discharge between said first and said second anode electrodes. r

6. A cathode ray tube comprising an envelope, a first anode electrode mounted within said envelope and adapted to be maintained at an operational potential, said anode electrode spaced from the wall of said envelope, 2. second anode electrode within said envelope comprising two portions insulatingly spaced from each other, one of said second anode portions arranged closer to said first anode electrode than the other of ,first anode electrode during tube operation, and

a conductive-means connecting said two second "anode portions, said conductive means including a resistance element of low conductivity for suppressing an electrical discharge between said first and second anode electrodes.

7. A cathode ray tube comprising an evacuated envelope, a first electrode mounted within said envelope and adapted to be maintained at an operational potential, a second electrode spaced from said first electrode and having an adjacent part enclosing a portion of said first electrode, lead means connected to said second electrode remote from said first electrode for connecting said second electrode to a source of higher potential relative to that maintained on said first electrode during tube operation, said adjacent enclosing part of said second electrode having high electrical resistance for suppressing electrical d'scharge between said first and second electrodes.

8. A cathode ray tube comprising an evacuated enve 9P a first electrode mounted within said envelope, said first electrode spaced from the wall of said envelope and adapted to be maintained at an operational potential, a second electrode including a conductive coating on the inner surface of said envelope wall, said coating having a portion enclosing part of said first electrode, lead means sealed through said envelope wall in contact with said conductive coating remote from said enclosing portion for connecting said second electrode to a, source of higher potential relative to that maintained on said first electrode during tube operation, said enclosing portion of said second electrode having a high electrical resistance for suppressing electrical discharge between said first and second electrodes.

9. A cathode ray tube including an evacuated envelope, electrodes Within said envelope for producing an electron beam, said electrodes comprising a first anode electrode spaced from the wall of said envelope and adapted to be maintained at an operational potential and a second anode electrode including a conductive coating on the inner surface of said envelope wall, a lead means sealed through said envelope wall in contact with one portion of said second anode coating for connecting said second anode to a. source of higher potential relative to that maintained on said first electrode during tube operation, said conductive second anode coating including another portion enclosing at least part of said first anode electrode, said enclosing portion of a material having a high electrical resistance to suppress electrical discharge between said first and second anode electrodes.

10. A cathode ray tube including an evacuated envelope, electrodes within said envelope for producing an electron beam, said electrodes comprising a first anode electrode spaced from the wall of said envelope and adapted to be maintained at an operational potential and a second anode electrode including a conductive coating on the inner surface of said envelope wall, a lead means sealed through said envelope wall in contact with one portion of said second anode coating for connecting said second anode to a source of higher potential relative to that maintained on said first electrode during tube operation, said conductive second anode coating including another portion spaced on said inner envelope wall surface from said one portion, and a material of high electrical resistivity deposited on the inner envelope surface between said second anode portions and electrically connected to said anode portions for suppressing an electrical discharge between said first and second anode electrodes.

11. A cathode ray tube including an evacuated envelope, electrodes within said envelope for producing an electron beam, said electrodes comprising a first anode electrode spaced from the wall of said envelope and adapted to be maintained at an operational potential and a second anode electrode including a conductive coating on the inner surface of said envelope wall, a lead means sealed through said envelope wall in contact with one portion of said second anode coating for connecting said secondanode to a source of higher potential relative to that maintained on said first electrode during tube operation, said conductive second anode coating in- ,luding another portion spaced on said inner tained at an operational potential, said first anode electrode spaced from the wall of said envelope, a second anode electrode including a conductive coating on the inner surface of said wall, lead means connected to one portion of said second anode coating for connecting said anode to a source of higher potential relative to that maintained on said first anode electrode during tube operation, and means of high electrical resistivity electrically connected to said second anode coating in the electrical path between said first anode electrode and said lead means.

13. A cathode ray tube comprising an evacuated envelope, a first electrode mounted within said envelope and adapted to be maintained at an operational potential, said first electrode spaced from the wall of said envelope, a second electrode including a conductive coating on the inner surface of said wall, lead means connected to one portion of said second electrode coating for connecting said second electrode to a source of higher potential relative to that maintained on said first anode electrode during tube operation, and means of high electrical resistivity connected to said second electrode coating in the electrical path between said first electrode and said lead means.

LLOYD E. SWEDLUND.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,123,636 Schwartz July 12, 1938 2,151,992 Schwartz Mar. 28, 1939 2,185,590 Epstein Jan. 2, 1940 2,232,083 Strohfeldt Feb. 18, 1941 2,269,115 Koch Jan. 6, 1942 2,409,514 Pratt Oct; 15, 1946 2,434,196 Cawein Jan. 6, 1948 

